1. Technical Field of the Invention
The present invention relates to a composite ceramic body, composed of a matrix of alumina particles to which nano-zirconia particles are dispersed, which is used for a gas sensor.
2. Related Art
An engine of a motor vehicle has an exhaust system on which a gas sensor is mounted for measuring an oxygen concentration or the like of exhaust gases. The gas sensor incorporates therein a gas sensing element composed of ceramic.
The gas sensing element has a structure with an external surface adapted to be brought into contact with exhaust gases. During startup of the engine, exhaust gases are emitted containing water droplets spattering across the gas sensing element.
Meanwhile, in order to activate the gas sensing element, the gas sensing element is heated to temperatures of, for instance, 600° C. or more in use.
If the water droplets adhere to a surface of the gas sensing element, these areas tend to be rapidly cooled, thereby suffering thermal shock. This results in a risk of cracks or chips occurring to the gas sensing element.
To avoid such thermal shock, there has been known a control of minimizing a temperature increase in the gas sensing element during the startup of the engine with increased likelihood of bearing sputtering water droplets as disclosed in Japanese Patent Application Publication No. 8-15213.
Further, there has been known a ceramic material, having increased strength and fracture toughness, which is composed of nano-composite material in which nano particles are dispersed in a matrix as disclosed in Japanese Patent No. 2703207.
However, minimizing the temperature increase of the gas sensing element during the startup of the engine results in a delay in activating the gas sensing element during the startup of the engine. Thus, there is a risk of degradation occurring to the detecting precision of exhaust gases during the startup of the engine.
Furthermore, since exhaust gases tend to contain harmful gas compounds during the startup of the engine, it is quite important to detect the oxygen concentration in exhaust gases during the startup of the engine for controlling an air/fuel ratio. In addition, minimizing the temperature increase of the gas sensing element during the startup of the engine results in a delay in activating the gas sensing element and such an expedient is unfavorable.
Although the temperature of the gas sensing element is desired to immediately rise up to an activating temperature from the beginning of the startup of the engine, there is a need to consider the risk of cracks (water-incursion cracks) occurring to the gas sensing element due to attached water droplets as set forth above.
In order to avoid the occurrence of water-incursion cracks, a ceramic material, forming the surface of the gas sensing element, needs to have strength withstanding thermal shock.
For the ceramic material having increased strength, it has been known to use nano-composite material as described above.
However, even if a composite ceramic body is made of materials using the nano-composite material, it has been difficult to obtain adequate strength.
With the nano-composite material, nano particles are hard to disperse with an increased risk of causing the nano particles to agglutinate, resulting in an increased likelihood of causing the formation of pores in the composite ceramic body.
As a result of dedicated studies conducted by the instant inventors, it has been found that sizes of the pores have correlations with particle diameters of the particles forming the matrix and the sizes and a volume of the pores in the composite ceramic body have an affect on strength.